Method and apparatus for mitigating feedback in a digital radio receiver

ABSTRACT

Embodiments of an acoustic feedback suppressor determine the energy in each of a plurality of frequency bands of frames of an audio signal. The energy in each of the plurality of frequency bands is compared to characteristic of human voice to determine that a present frame contains content that is not likely human voice and exhibits a characteristic of feedback. Upon determining that feedback is occurring, an adaptive gain reduction is applied to the band in which feedback is suspected to be occurring.

This application is a National Stage filing under 35 USC §371 ofco-pending Patent Cooperation Treaty international application havingSerial No. PCT/CN2012/086873 (the ‘PCT international application’) filedon Dec. 18, 2012. This application claims priority to the PCTinternational application, the entire contents of which are incorporatedherein by reference.

FIELD OF THE DISCLOSURE

The present disclosure relates generally to mitigating feedback, andmore particularly to detecting the onset of acoustic-sourced feedback ina digital radio receiver and applying suppressive filtering to mitigateand reduce the feedback.

BACKGROUND

Feedback in audio systems is caused by the output signal coupling to aninput through an acoustic path, creating a regenerative signal in theloop that results in an undesirable sound. It occurs in all types ofsystems, from simple public address systems to sophisticated wirelessradio communication systems. The conventional ways of dealing withfeedback sound include several approaches. One approach is physicallyseparating the input (e.g. microphone) and the output (e.g. speaker)sufficiently to prevent the regenerative signal from occurring. Ofcourse, this is not always possible. Another approach is to simply applya notch filter at the feedback frequency. This approach is onlyeffective if the feedback occurs at a known frequency, or if thefrequency is determined and then a corresponding filter applied, and ifthe filter doesn't unduly affect the frequency content of desiredsignals. However, in more complex systems there can be various types offeedback occurring at different frequencies, with differentcharacteristics. Pre-emptively inserting filters to address all forms offeedback would not be practical. Since feedback is regenerative, anotherapproach that is often used is to vary the pitch of audio signals toprevent the regenerative effect from occurring. Pitch can either beshifted to avoid a known resonance frequency or frequency band, or thefundamental pitch of a sound can be determined and varied above andbelow the fundamental pitch. Pitch shifting is effective, but can oftenbe detected by a listener and is thus not an optimum solution. Othermethods include simply detecting a strong tone-like signal in the audiospectrum of the signal through the system and responding by loweringvolume, or applying a notch filter at the feedback frequency. This,however, presumes feedback is occurring, which means that listeners arehearing feedback by the time the system detects and applies a measure toaddress the feedback.

All of the conventional methods are effective, but have their drawbacks.They either require identifying feedback as it occurs, meaning it isbeing heard by the time it is detected, or requires foreknowledge as tothe frequency so that a selected filter can be applied.

Accordingly, there is a need for a method and apparatus for suppressingfeedback in an audio system that operates faster than conventionalmethods and in an adaptive manner to address different kinds offeedback.

BRIEF DESCRIPTION OF THE FIGURES

The accompanying figures, where like reference numerals refer toidentical or functionally similar elements throughout the separateviews, together with the detailed description below, are incorporated inand form part of the specification, and serve to further illustrateembodiments of concepts that include the claimed invention, and explainvarious principles and advantages of those embodiments.

FIG. 1 is a block diagram of a transmitting device and receiving devicein accordance with some embodiments;

FIG. 2 is a functional block diagram of a feedback suppressor inaccordance with some embodiments;

FIG. 3 is a flow chart diagram of a method for detecting feedback in amid-band frequency region of a signal in accordance with someembodiments;

FIG. 4 is a flow chart diagram of a method for detecting feedback in alow-band frequency region of a signal in accordance with someembodiments;

FIG. 5 is a functional block diagram of a feedback suppressor inaccordance with some embodiments;

FIG. 6 is a index chart used by a feedback suppressor in accordance withsome embodiments;

FIG. 7 is a series of energy indexed histograms derived from the indexchart of FIG. 6 at several times; and

FIG. 8 is a flow chart diagram of a method of performing feedbackdetection in accordance with some embodiments.

Skilled artisans will appreciate that elements in the figures areillustrated for simplicity and clarity and have not necessarily beendrawn to scale. For example, the dimensions of some of the elements inthe figures may be exaggerated relative to other elements to help toimprove understanding of embodiments of the present invention.

The apparatus and method components have been represented whereappropriate by conventional symbols in the drawings, showing only thosespecific details that are pertinent to understanding the embodiments ofthe present invention so as not to obscure the disclosure with detailsthat will be readily apparent to those of ordinary skill in the arthaving the benefit of the description herein.

DETAILED DESCRIPTION

Embodiments include a method for mitigating feedback in a radio receiverthat includes various processes and steps, such as receiving, at theradio receiver, a radio signal and generating a digital audio signalfrom the radio signal. The digital audio signal is formatted into aseries of frames. The method further includes determining an energylevel in each of a plurality of frequency bands of the digital audiosignal and calculating at least one ratio of a signal energy level of afirst frequency band of the plurality of frequency bands to a signalenergy level of a second frequency band of the plurality of frequencybands. The method further includes determining that the at least oneratio exceeds a threshold and determining, based at least in part upondetermining that the at least one ratio is not consistent with a voicesignal, that feedback is occurring in one of the plurality of frequencybands, and applying a gain reduction to the frequency band in whichfeedback is occurring.

applying a gain reduction to the frequency band in which feedback isoccurring. FIG. 1 is a block diagram 100 of a transmitting device 102and receiving device 104 in accordance with some embodiments. Thetransmitting device 102 and the receiving device 104 are both two-wayradio devices such as those commonly used by public safety and othersuch personnel. A two-way radio operates in a half-duplex mode; the usercan either speak (transmit) or listen (receive) but not both at the sametime (e.g. full duplex). Two-way radios can communicate directly witheach other, without using a network, and can also be trunked which usesa networked infrastructure, repeaters, and other equipment. Thetransmitting device 102 shows only the transmitter line up, while thereceiving device 104 only shows the receiver line up. Both devices,however, will contain both the transmitter and receiver components shownhere for transmitting device 102, and receiving device 104.

In the transmitting device 102, a microphone 106 receives acoustic soundsignals which are converted by the microphone 106 into an electricalanalog signal that is amplified by the microphone amplifier 110. Theamplified analog signal is then digitized by an analog to digitalconverter 110. An uplink audio processor block 112 formats the digitalsignal produced by the analog to digital converter 110 for transmitting,such as by grouping digital samples into frames which are then processedto reduce the noise in the samples by a noise suppressor block 114. Theprocessed audio samples output from the noise suppressor 114 are thenspeech encoded by a speech encoder 116. The speech encoder 116 modelsspeech into a set of parameters, as is done, for example, using vectorsum excited linear predictive (VSELP) or advanced multi-band excitation(AMBE) modeling. The encoded speech signal produced by the speechencoder 116 is provided to a transmitter 118 that modulates andtransmits the signal over a radio channel 120. The radio channel 120 canbe a direct channel between the transmitting device 102 and thereceiving device 104 or it can be through a network, repeater, or othersuch equipment that relays the signal from the transmitting device 102to the receiving device 104.

The receiving device 104 receives the data over the radio channel 120 ata receiver 122. The receiver 122 contains a demodulator that demodulatesthe received signal to obtain the speech encoded signal that wastransmitted by the transmitting device 102. The receiver 122 providesthe received encoded signal to a speech decoder 124 that applies aspeech decoding process to the signal provided by the receiver 122. Thespeech decoder 124 essentially reverses the encoding performed by speechencoder 116 to produce a digital audio signal. The digital audio signalcan be organized in frames of audio samples which are processed by anacoustic feedback suppressor 126. The acoustic feedback suppressor 126processes the received audio samples to detect signals that do not havethe typical characteristics of speech, and which appear likely to be theonset of a regenerative signal component. The acoustic feedbacksuppressor 126 determines the energy of the audio signal in variousfrequency bands and can apply various tests to determine if the audiosignal has energy content that is distributed in the frequency spectrumof the audio signal in a way that resembles speech, both in an instantand over time. When the audio signal energy distribution with respect tofrequency is distributed such that is it unlikely to be speech, or when,over time, the energy distribution fails to behave like speech, theacoustic feedback suppressor 126 will reduce the gain in one or morefrequency bands. As the signal continues to be received, the gainreduction continues to be applied over time, until the non-speech energyis sufficiently diminished. If the audio signal appears to be speech,though, the acoustic feedback suppressor 126 does not modify the signal.

The output of the acoustic feedback suppressor 126 is provided to adownlink audio processing block 128 which can include equalizers,adaptive/fixed gain controllers, an intelligibility booster, and localsound generators, such as tone, voice announcement and comfortable noisegenerators that are used to format the digital audio signal forconversion to analog. The D/A conversion is achieved using a digital toanalog converter 130. The digital to analog converter 130 produces ananalog audio signal which is amplified to a selected volume level byoutput amplifier 132. The output amplifier applies the amplified analogaudio signal to a speaker 134. The speaker converts the electricalanalog audio signal into an acoustic signal so that a nearby listenercan hear the acoustic signal. If the receiving device 104 issufficiently close to the transmitting device 102, the speaker output136 can be received at the microphone 106 of the transmitting device,which can result in regenerative feedback. The feedback can rapidlyincrease as the sound loops through the transmitting device 102 and thereceiving device 104. However, the acoustic feedback suppressor 126detects audio energy that indicates the audio signal is not likelyspeech, or includes energy that is not likely speech, and applies anadaptive gain reduction to frequency bands that appear to beexperiencing the onset of regenerative feedback.

It is known that the energy in human voiced speech signals tends to begreatest under 1500 Hz. The fundamental frequency of adult male speechis in the region of 125 Hz, adult female speech has a fundamentalfrequency around 200 Hz, while child speech varies around 250 Hz to 400Hz. Above about 1400 Hz, the average energy (over time) for voicedspeech decreases at about 6 dB per decade. Unvoiced sounds, likefricatives in consonants, have significant energy at higher frequencies,but are relatively short in duration. Furthermore, the frequencyresponse of audio components commonly used in devices such as two-wayradios tends to peak at high frequencies, significantly above thosewhere most of the energy in human speech occurs. Accordingly, ingeneral, audio signals that tend to have higher energy content in higherfrequency ranges than in lower frequencies are not consistent withspeech, and are likely to be the result of feedback. Furthermore, thepeak energy in human speech shifts over time as the speaker speaks andpronounces different sounds and words. Accordingly, if the energy in midand/or low frequency bands is consistent at the same frequency over aperiod of time where it would be unlikely to be speech, feedback may beoccurring. Knowing the characteristic of typical human speech allows theembodiments taught herein to detect audio signals that do not appear tobe speech and which are consistent with feedback. An adaptive gainreduction can then be applied to mitigate whatever feedback may beoccurring. As shown here the acoustic noise suppressor 126 is in thereceiver path 104 of a radio device, but it will be appreciated by thoseskilled in the art that it can be equivalently placed in the transmitpath 102.

FIG. 2 is a functional block diagram of a feedback suppressor 200 inaccordance with some embodiments. The feedback suppressor 200 can beused as the acoustic feedback suppressor 126 of FIG. 1 in someembodiments. The various processes performed by the feedback suppressor200 are abstracted into boxes in FIG. 2. Each process can beimplemented, for example, by a digital signal processor executingappropriate instruction code designed to perform the abstractedprocesses.

A speech decoder 202 receives encoded speech 204 from a demodulator of aradio receiver (e.g. receiver 122). The speech decoder 202 decodes thespeech to produce a digital audio signal that is provided to both anadaptive sensitivity calculation block 206 and a windowing block 208.The adaptive sensitivity calculation block 206 determines thefundamental frequency or pitch of the audio signal, such as by using,for example, an average magnitude difference function. The fundamentalfrequency is used to dynamically select thresholds for a high bandfeedback detector 224. The pitch is determined on voiced speech.

The windowing block 208 formats the decoded speech into windows orframes of samples. In some embodiments the windowing block 208 producesframes of 10 millisecond (ms) length, having 80 (eighty) samples of thedecoded speech signal having 24 (twenty four) buffer samples before andafter the 80 (eighty)samples of the decoded speech to produce 128 pointframes with 8 KHz sampling rate. Each frame is processed on a frame byframe basis. The frames are processed by a time to frequency conversionblock 210 (e.g. digital Fourier transform) to produce a frequencyspectrum for each frame which is provided to a band energy calculationblock 212. The band energy calculation block 212 determines the energyin each of a plurality of defined frequency bands. For example, thefrequency bands can be defined as a very low band 214 that goes from thelowest frequency (e.g. 0 Hz) of the receiver to 500 Hz, a low band 216that goes from the lowest frequency to 1400 Hz, or to a frequency in arange such as 1000 to 1400 Hz. The frequency bands can further include amid band 218 from 500 Hz to 2000 Hz, a high band 220 from 1400 Hz to thehighest frequency of the receiver, such as 4000 Hz, and a very high band222 from 2000 Hz to the highest frequency. The size of the frequencybands 214-222 as shown here are not meant to be proportional to energycontent or magnitude, and are arranged here only to show their range andhow they can overlap.

For each band 214-222 the band energy calculation block 212 determinesthe magnitude of the energy contained in the respective band. The bandenergy calculations are provided to each of several band feedbackdetector blocks, which include the high band feedback detector block224, the mid band feedback detector block 226, and the low band feedbackdetector block 228.

The high band feedback detector 224 uses the output of the adaptivesensitivity calculation 206 to determine appropriate thresholds based onthe pitch of voiced speech in the audio signal. Two ratios aredetermined; the ratio of the energy in the low band 216 to the energy inthe high band 220, and the ratio of the energy in the very low band 214to the energy in the very high band 222. Since the energy in humanspeech is mostly in the lower frequencies, if these ratios are too lowit indicates the audio signal contains unusually high frequency contentthat is uncharacteristic of speech, and may therefore be feedback in thehigh bands. The ratio of the energy in the low band 216 to the energy inthe high band 220 is compared to a first threshold, and the ratio of theenergy in the very low band 214 to the energy in the very high band 222is compared to a second threshold, where the first and second thresholdsare based on the determined pitch.

The mid band feedback detector 226 also determines two energy ratios,the ratio of energy in the mid band 218 to the energy in the very highband 222, and the energy in the mid band 218 to the energy in the verylow band 214. The operation of the mid band feedback detector 226 isshown in FIG. 3, which is a flow chart diagram of a method 300 fordetecting feedback in a mid-band frequency region of a signal inaccordance with some embodiments. At the start 302 the two mid bandenergy ratios have been calculated for a present frame (mid to very highand mid to very low). The method 300 can determine whether there is weakmid-band feedback occurring, as in process 304, by comparing the mid tovery low and mid to very high energy ratios to mid-band weak lowthreshold and a mid-band weak high threshold, respectively. If theratios are larger than these thresholds, then weak mid-band feedback maybe occurring, and a counter for weak feedback is incremented in process306. The counter value is then evaluated in process 308. The counterindicates whether the weak mid-band feedback has been occurring for asufficient period as it is incremented with each successive frame thatthe mid band ratios fall under their respective thresholds. If the midband ratios do not fall under their respective thresholds, the counteris reset, as in process 318. If the counter indicates that thepreselected number of consecutive frames have elapsed where the mid bandratios fall under their respective thresholds, then a weak mid band flagcan be set in process 310. The method 300 also determines if there isstrong mid band feedback occurring in process 312. The mid band ratiosare compared to mid band strong low and mid band strong high thresholds,which are lower than the mid band weak low and mid band weak strongthresholds, respectively. If the mid band energy ratios fall under themid band strong thresholds, the strong mid band feedback flag is set inprocess 314. Once the weak and strong mid band feedback determinationshave been made, the method 300 terminates 316.

Returning to FIG. 2, the feedback suppressor 200 also includes the lowband feedback detection block 228. Low band feedback, which occurs under500 Hz (the very low band 214), can be difficult to detect becausespeech tends to have significant energy in the low band region. However,speech also varies with time, so while speech energy is high in the lowband, it changes in magnitude over time, unlike feedback which tends tobe consistent. Therefore, to detect low band feedback, the energyconsistency over time must be examined. A method for performing low bandfeedback detection is shown in FIG. 4, which shows a flow chart diagramof a method 400 for detecting feedback in a low-band frequency region ofa signal in accordance with some embodiments. At the start 402, the lowband feedback block 228 has calculated the signal to noise ratio (SNR)in the low band for several consecutive frames. The method 400 thencompares the present SNR (SNR(n)) with the SNR of the most recent (orother recent) frame (SNR(n−1)), in process 404. If the differencebetween the present and recent SNR level is less than a preselectedamount (e.g. “yes” out of process 404), then an SNR threshold counter isincremented in process 406, otherwise the SNR threshold counter is resetin process 414. When the SNR threshold counter is incremented, the countvalue of the SNR threshold counter is evaluated in process 408. If theSNR increment counter meets a preselected value, which corresponds to atime duration which would indicate that the energy in the low band hasbeen consistent, rather than varying as speech would, then low bandfeedback has been detected, as indicated in process 410, where a flagcan be set for further operations. The method then terminates 412 forthe present frame. The SNR can be determined by a variety of knownmeans. In some embodiments the SNR can be determined as 10 log(maximumlow band energy/minimum low band energy).

Referring again to FIG. 2, once the high, mid, and low band feedbackdetection processes 224, 226, and 228, respectively, have been performedfor the present frame, further processing is performed. A voice activitydetector (VAD) block 230 determines whether the present frame containsvoice activity. Voice activity can be determined based on thetime-smoothed maximum and minimum energies in the very low band 214. Ifthe difference between the maximum energy and the minimum energy in thevery low band 214 is greater than a VAD threshold, then a VAD flag canbe set, indicating the present frame contains speech, otherwise the flagis cleared indicating there is no speech in the present frame. Whenthere is no speech present and indicated by the VAD 230, then all gainadjustments can be reset to a normalized level.

A sub-band gain generator 232 determines the gain to be applied to thevarious bands, 214, 218, 222. An overall gain can be determined as theproduct of the two ratios determined in the high band feedback detectorblock 224, which are the ratio of the energies of the low band 216 tothe high band 220 and the very low band 214 to the very high band 222.The smaller these ratios, the more likely there is feedback occurring.The gain can further be smoothed by taking into account the gain used inthe most recent frame along with a smoothing value. This allows for thegain to ramp up, as well, when speech is detected as indicated by theVAD 230. In the gain multiplier block 234, the new gain factors areapplied to their respective bands. The gain multiplied frame is thenconverted back into the time domain in frequency to time conversionblock 236 and the output 238 is a signal that has been examined forfeedback, and when detected, mitigation measures have been applied toreduce the effect of feedback.

FIG. 5 is a functional block diagram of a feedback detector 500 inaccordance with some embodiments. The feedback detector 500 can be usedas an additional or alternative processing element in the acousticfeedback suppressor 126 of FIG. 1 in some embodiments. In someembodiments it can be used to supplement and help verify the feedbackdetection processes performed by the feedback suppressor described inFIGS. 2-4. The speech decoder 502 can operate as the speech decoder 124of FIG. 1, and decodes received encoded speech to produce a digitalaudio signal which is windowed 504 to produce a succession of frames.Each frame contains a plurality of time-based samples for consideration,and may be padded with zeros to produce a frame of a pre-determinedlength. A frequency representation (frequency frame) of each time frameis produced by, for example, a fast Fourier transform block 506. Thefrequency frame is then processed to determine the energy in each of aplurality of frequency bands or filterbank channels in process 508. Insome embodiments the energies can be determined for 16 (sixteen)frequency bands or filterbank channels distributed along a Barkfrequency scale, although other frequency scales can be used, as well.Upon determining the filterbank energies, the highest filterbank channelenergy is stored (maxFBE) and at least the three channels having thehighest energies are ranked (e.g. 1^(st), 2^(nd), and 3^(rd) highest andcorresponding indices, e.g. 1 through 16) in process 510. These rankedresults are stored in a table, array, or other data structure along withtheir corresponding channel indices (maxFBEI[i][j], i=1, . . . , 16, j=1energy, j=2 index) with the lowest index (i=1) corresponding to thechannel with the highest energy. Additional, or even all otherfilterbank energies, can be likewise rank ordered. Furthermore, theaverage long and short term energies are calculated in process 522. Theshort term energy is the total energy of the present frame (ASTE),whereas the average long term energy (ALTE) is the average of a numberof previous frames over a period of time, such as, for example 1 second.

The filterbank channel energy rankings of process 510 can be used toform a histogram to track the changes in rankings of the filterbankchannels over a period of time in process 512. An accumulated count ofhow often a particular channel contains the maximum energy in a frame isstored in a bin (e.g. one of 16 bins) corresponding to a filterbankchannel in the histogram table. The count in each histogram bin istime-weighted so that the histogram represents the filterbank maximumenergy occurrence distribution over a limited, moving time window. Thelength of the time window is nominally between 0.15 and 1.0 seconds,corresponding to the duration of a typical speech syllable. From thehistogram a set of probabilities for the maximum energy occurring ineach filterbank channel (p[i]), and the expected values (pev[i]) arecalculated in process 514 using common statistical procedures. Thefilterbank channel energy probabilities along with their indices(maxFBEP[i][1,2], i=1, . . . , 16, j=1 probability, j=2 index) are thenranked ordered from highest to lowest so that the channels having thehighest probabilities of containing the maximum energy may be easilyidentified in process 516 with the lowest index (1=1) corresponding tothe channel with the highest energy probability. The band(s) containingfeedback usually exhibit the highest, second, or third highest energyprobabilities. In addition, energy duration can be used as a parameterto detect possible howling because the duration of a feedback event isusually longer than that of a syllabic speech sound, and because itsband residency is more stationary. The maximum acoustic energy of speechtends to vary more in time and in frequency than does feedback energy.The fact that feedback generally has a longer duration in a givenfrequency band or adjacent bands can be used to further discriminatebetween normal speech and the presence of feedback. Minimum durationlimits can be set for detecting a feedback condition where the limitsare shorter for higher bands than in lower bands. Usually the highestenergy sustained speech sounds are voiced sounds occurring at lowerfrequencies so longer dwell times must be assigned to differentiatefeedback. Accordingly, dwell times for each filterbank band having themaximum energy and maximum energy probability can be determined inprocess 518. This is accomplished by keeping a count of consecutiveframes for each filterbank channel that exhibits the 1^(st) 2^(nd), or3^(rd) highest probability of having the highest energy, and having thehighest energy. If the channel does not meet the criteria the count isreset to zero. These dwell counts are stored in a memory array,maxEcnt[i] and maxPcnt[i] where i is the index of a specific filterbankchannel.

In process 520 the percentage of low band energy (elbPRCNT), forexample, in one embodiment the band from 80-300 Hz, of the total frameenergy is determined, and in process 524 a high to low band energy ratio(hblbRatio), is determined, where the total energy in the higherfilterbank channels above 500 Hz is divided by the energy in thechannels below 500 Hz. Note that the limits of these frequency bands maybe adjusted dependent on the total frequency response of the decodedoutput speech from FIG. 1 block 124. Different audio pre-filteringlineups and codec responses may alter the overall bandpass at thedecoder output, FIG. 1, block 124. The various parameters and ratios areall evaluated by a feedback detector logic block 526. The feedbackdetector logic consists of a sequence of comparison tests in which thesignal-derived parameters maxEcnt[i], maxPcnt[i], maxFBE, ALTE,elbPRCNT, hblbRatio, maxFBEI[i][j], and maxFBEP[i][j] are comparedagainst empirically derived threshold constants. If the logical resultof the comparisons is true a howling detection flag (HowlDetFlg) is setto logical 1. If not the flag is set to 0. The output of the feedbackdetector logic 526 is an indication of whether feedback is occurring,and if so, in what band or bands, which is evaluated in process 528.When feedback is occurring, as indicated, for example, by a flag set bythe feedback detector logic 526 to indicate such, the suppression can beapplied in process 530 that is adapted to the detected feedback.Generally, a gain reduction factor is determined, and applied to one ormore of the filterbank channels (0-15) to modify each frame in thefrequency domain. After applying the gain reduction, if any is needed,the frequency frame is converted back into a processed time frame usingan inverse Fourier operation, and the processed time frame is thenforwarded to, for example, the downlink audio processing block 128 ofFIG. 1.

A flowchart of one embodiment of the feedback detector logic in FIG. 5block 526 is shown in FIG. 8. Note that the threshold parameters shownin FIG. 8 are only representative and different embodiments may usedifferent parameters. The HowlDetFlg feedback present flag and indicesof the filterbank channels in which it most likely occurs are madeavailable in process 528. In one embodiment this information is passedon to an embodiment of the feedback suppressor as exemplified in FIG. 2,and in particular blocks 224, 226, and 228 and can be used to supplementor supersede the feedback detection performed in these processes. Inanother embodiment, the howling detector of FIGS. 5 and 8 may useanother means to suppress and attenuate the feedback in specificfilterbank channels as in process 530, independent of the methods usedin embodiment of the feedback suppressor depicted in FIGS. 2-4.

FIG. 6 is an index chart 600 used by a feedback suppressor in accordancewith some embodiments. The chart 600 indexes the frequency bands (rows)or filterbank channels having the highest three energies with frequencyincreasing along the vertical axis 602. Each column corresponds to aframe of information at regular periods, with time increasing along thehorizontal axis 604. The highest energy is denoted with a filled circle,the second highest with a gray circle, and the third highest with anwhite circle. The chart 600 charts a sound burst that commences asvoiced speech and then becomes feedback in the high band. From theorigin to period 606 the highest energy bands are concentrated in thelower frequency bands. Between periods 606 and 608, the energy in thelow bands diminishes, as would be consistent with a sound thatterminates with unvoiced speech, where there is higher energyconcentrated in the mid band frequencies. From period 608 to period 610,and thereafter, the energy is concentrated in the higher bands, and thehighest energy is persistently in one filterbank channel 612, as isconsistent with feedback or howling.

FIG. 7 is a series of energy indexed histograms 700 derived from theindex chart of FIG. 6 at several times. Each histogram charts frequencyalong their horizontal axis, in discrete divisions corresponding to thefilterbank channels along the vertical axis of FIG. 6, and a count ofranking over a period of time (e.g. multiple frames), based on theranking performed by, for example, process 510 in FIG. 5, is chartedalong the vertical axis. In some embodiments, the histogram can beproduced by summing the energy in each filterbank channel of each framefor a period of time. Histogram 702 can represent the energydistribution from the origin to period 606 in FIG. 6. The energy isconcentrated in the lower filterbank channels, as is consistent withvoiced speech. In histogram 704, the energy is more centrallydistributed, as would be consistent, for example, in the time betweenperiods 606 and 608 of FIG. 6. When feedback, such as howling, isoccurring, as in the time between periods 608 and 610 of FIG. 6, theenergy tends to be concentrated in fewer filter channels as indicated Ihistogram 706.

FIG. 8 is a flow chart diagram of a method 800 of performing feedbacksuppression in accordance with some embodiments. The method 800 canoperate as the feedback detector logic 526 of FIG. 5. At the start 802,a present frame under consideration has been converted to a frequencyframe. Furthermore, there it can be assumed in the present example thatthe process has been executed for several frames in succession prior tothe present frame so that there is a history in the various parametersto be determined. Accordingly, the various parameters are determined, asin process 804, including the percentage of energy in the low bandrelative to the total energy (elbPRCNT), the average long term energy(ALTE) of the past n consecutive frames, the short term energy of thepresent frame (ASTE), the ratio of high band energy to low band energy(bhlbRatio), the maximum filter band energy index (maxFBE[i][j]), themaximum filterbank energy (maxFBE), the count of successive frames of afilterbank channel having maximum energy (maxEcnt[i]), and the count ofsuccessive frames of a filterbank channel having the highest probabilityof having the highest energy (maxPcnt[i]).

A high band evaluation is performed in process 812. The evaluation 812determines whether either maxEcnt[i] or maxPcnt[i] exceed 20 frames (inthe present example), whether the index number is 5 or higher (ensuringonly the higher frequencies are used in this evaluation), and whetherthe maximum filterbank energy is greater than the average long termenergy. If this evaluation is false, meaning at least one condition isnot true, the high band (HB) persistence flag is cleared in process 814.If the high band evaluation is true, meaning all conditions are true,then the high band persistence flag is set (or not cleared if alreadyset) in process 816.

A low band evaluation is performed in process 806. The evaluation 806determines whether either maxEcnt[i] exceeds 50 successive frames or ifmaxPcnt[i] exceed 100 successive frames (in the present example),whether the index number is lower than 5 (ensuring only the lowerfrequencies are used in this evaluation), and whether the maximumfilterbank energy is greater than the average long term energy. If thisevaluation is false, meaning at least one condition is not true, the lowband (LB) persistence flag is cleared in process 810. If the low bandevaluation is true, meaning all conditions are true, then the low bandpersistence flag is set (or not cleared if already set) in process 808.

A high band feedback detection evaluation is performed in process 818,which determines whether the maximum filterbank index probability isgreater than 3 (in this example) or whether the high band energy to lowband energy ratio is greater than 2, and whether the high bandpersistence flag is set, and whether the maximum filterbank energy isgreater than 2% of the average long term energy. If the high bandfeedback detection evaluation 818 is true, then the feedback detectedflag is set in process 824 and the method can terminate 826. If the highband feedback detection evaluation 818 is false, meaning at least one ofthe conditions is not true, the feedback detected flag is not set, andthe low band feedback detection evaluation 820 is performed.

A low band feedback detection evaluation is performed in process 820,which determines whether the low band persistence flag is set, whetherthe low band persistence flag is set, whether the energy in the low bandis at least 20% of the total frame energy, and whether the maximumfilterbank energy is at least 2% of the average long-term energy. If thelow band feedback detection evaluation 820 is true, meaning allconditions are true, then the feedback detected flag is set in process824. If the low band feedback detection evaluation 820 is false, meaningat least one condition is not true, then the feedback detected flag iscleared in process 822 and the method terminates 826.

Once the method 800 terminates 826, if the feedback detected flag isset, then an appropriate gain reduction can be applied to the frequencyframe before converting it back to the time domain. Since the feedbackdetection and gain reduction are applied over successive frames, thefrequency bands in which the feedback has been detected will continue todiminish until the feedback conditions are longer evident. Due to theregenerative nature of feedback, the gain reduction propagates throughthe feedback loop to eliminate the feedback. Once the feedbackconditions are no longer met, the gain can be ramped back up to normallevels, or until the feedback conditions are evident again.

Embodiments as exemplified herein have the benefit of quicklyidentifying feedback, such as howling, but evaluating a received audiosignal against characteristics of speech, such as having an unusuallyhigh energy in a high band compared to a low band, having low bandenergy that is both persistent and consistent in form, rather than morevariable like speech.

In the foregoing specification, specific embodiments have beendescribed. However, one of ordinary skill in the art appreciates thatvarious modifications and changes can be made without departing from thescope of the invention as set forth in the claims below. Accordingly,the specification and figures are to be regarded in an illustrativerather than a restrictive sense, and all such modifications are intendedto be included within the scope of present teachings.

The benefits, advantages, solutions to problems, and any element(s) thatmay cause any benefit, advantage, or solution to occur or become morepronounced are not to be construed as a critical, required, or essentialfeatures or elements of any or all the claims. The invention is definedsolely by the appended claims including any amendments made during thependency of this application and all equivalents of those claims asissued.

Moreover in this document, relational terms such as first and second,top and bottom, and the like may be used solely to distinguish oneentity or action from another entity or action without necessarilyrequiring or implying any actual such relationship or order between suchentities or actions. The terms “comprises,” “comprising,” “has”,“having,” “includes”, “including,” “contains”, “containing” or any othervariation thereof, are intended to cover a non-exclusive inclusion, suchthat a process, method, article, or apparatus that comprises, has,includes, contains a list of elements does not include only thoseelements but may include other elements not expressly listed or inherentto such process, method, article, or apparatus. An element proceeded by“comprises . . . a”, “has . . . a”, “includes . . . a”, “contains . . .a” does not, without more constraints, preclude the existence ofadditional identical elements in the process, method, article, orapparatus that comprises, has, includes, contains the element. The terms“a” and “an” are defined as one or more unless explicitly statedotherwise herein. The terms “substantially”, “essentially”,“approximately”, “about” or any other version thereof, are defined asbeing close to as understood by one of ordinary skill in the art, and inone non-limiting embodiment the term is defined to be within 10%, inanother embodiment within 5%, in another embodiment within 1% and inanother embodiment within 0.5%. The term “coupled” as used herein isdefined as connected, although not necessarily directly and notnecessarily mechanically. A device or structure that is “configured” ina certain way is configured in at least that way, but may also beconfigured in ways that are not listed.

It will be appreciated that some embodiments may be comprised of one ormore generic or specialized processors (or “processing devices”) such asmicroprocessors, digital signal processors, customized processors andfield programmable gate arrays (FPGAs) and unique stored programinstructions (including both software and firmware) that control the oneor more processors to implement, in conjunction with certainnon-processor circuits, some, most, or all of the functions of themethod and/or apparatus described herein. Alternatively, some or allfunctions could be implemented by a state machine that has no storedprogram instructions, or in one or more application specific integratedcircuits (ASICs), in which each function or some combinations of certainof the functions are implemented as custom logic. Of course, acombination of the two approaches could be used.

Moreover, an embodiment can be implemented as a computer-readablestorage medium having computer readable code stored thereon forprogramming a computer (e.g., comprising a processor) to perform amethod as described and claimed herein. Examples of suchcomputer-readable storage mediums include, but are not limited to, ahard disk, a CD-ROM, an optical storage device, a magnetic storagedevice, a ROM (Read Only Memory), a PROM (Programmable Read OnlyMemory), an EPROM (Erasable Programmable Read Only Memory), an EEPROM(Electrically Erasable Programmable Read Only Memory) and a Flashmemory. Further, it is expected that one of ordinary skill,notwithstanding possibly significant effort and many design choicesmotivated by, for example, available time, current technology, andeconomic considerations, when guided by the concepts and principlesdisclosed herein will be readily capable of generating such softwareinstructions and programs and ICs with minimal experimentation.

The Abstract of the Disclosure is provided to allow the reader toquickly ascertain the nature of the technical disclosure. It issubmitted with the understanding that it will not be used to interpretor limit the scope or meaning of the claims. In addition, in theforegoing Detailed Description, it can be seen that various features aregrouped together in various embodiments for the purpose of streamliningthe disclosure. This method of disclosure is not to be interpreted asreflecting an intention that the claimed embodiments require morefeatures than are expressly recited in each claim. Rather, as thefollowing claims reflect, inventive subject matter lies in less than allfeatures of a single disclosed embodiment. Thus the following claims arehereby incorporated into the Detailed Description, with each claimstanding on its own as a separately claimed subject matter.

We claim:
 1. A method for mitigating feedback in a two-way radio,comprising: receiving, an audio signal in a receiver path of the two-wayradio; mitigating feedback in the receiver path of the two-way radiousing a digital signal processor performing the following: generating adigital audio signal from the audio signal, the digital audio signalformatted in a series of frames; determining an energy level in each ofa plurality of frequency bands of the digital audio signal; calculatingat least one ratio of a signal energy level of a first frequency band ofthe plurality of frequency bands to a signal energy level of a secondfrequency band of the plurality of frequency bands; determining that theat least one ratio exceeds a threshold; and determining, based at leastin part upon determining that the at least one ratio is not consistentwith a voice signal, that feedback is occurring in one of the pluralityof frequency bands in the receiver path of the receiving two-way radiocaused by a transmitting two-way radio; and applying a gain reduction tothe frequency band in which feedback is occurring to suppress thefeedback.
 2. The method of claim 1, wherein determining the energy levelin each of the plurality of frequency bands comprises determining theenergy level in each of the plurality of frequency bands on a frame byframe basis.
 3. The method of claim 1, wherein determining the energylevel in each of the plurality of frequency bands comprises smoothingthe energy level over time with a smoothing factor.
 4. The method ofclaim 1, further comprising, dynamically setting the threshold based ona present pitch of the audio signal.
 5. The method of claim 1, whereindetermining the energy level in each of the plurality of frequency bandscomprises determining the energy level in two adjacent frequency bands.6. The method of claim 5, wherein the two adjacent frequency bands are ahigh and a low band, where the low band begins at a lowest frequency forthe receiver, and the high band ends a highest frequency of the receiverand the high and low bands are adjacent at a middle frequency.
 7. Themethod of claim 6, further comprising: determining an energy level in avery low band that is from the lowest frequency for the receiver to alow middle frequency that is below the middle frequency; determining anenergy level in a very high band that is from a high middle frequencythat is above the middle frequency to the highest frequency of thereceiver; determining a ratio of the energy in the very low band to thevery high band; and applying a gain reduction in the high band when theenergy in the very low band to the very high band exceeds a very low tovery high threshold.
 8. The method of claim 7, further comprising:determining the gain reduction based on the product of the ratio of theenergy in the very low band to the very high band and the ratio of theenergy in the low band to the energy in the high band.
 9. The method ofclaim 7, further comprising: determining an energy level in a mid band,where the mid band is from the low middle frequency to the high middlefrequency; determining a ratio of the energy in the middle band to theenergy in the very high band; determining a ratio of the energy in themiddle band to the energy in the very low band; and applying the gainreduction to the mid band when the ratio of the energy in the middleband to the energy in the very high band exceeds a strong mid to highthreshold and the ratio of the energy in the middle band to the energyin the very low band exceeds a strong mid to low threshold.
 10. Themethod of claim 9, further comprising: determining an energy level in amid band, where the mid band is from the low middle frequency to thehigh middle frequency; determining a ratio of the energy in the middleband to the energy in the very high band; determining a ratio of theenergy in the middle band to the energy in the very low band; andapplying the gain reduction to the mid band when the ratio of the energyin the middle band to the energy in the very high band exceeds a weakmid to high threshold and the ratio of the energy in the middle band tothe energy in the very low band exceeds a weak mid to low threshold fora preselected number of consecutive frames.
 11. The method of claim 1,wherein determining the energy level in each of the plurality offrequency bands of the digital audio signal comprises determining theenergy level in each of a plurality of consecutive frequency bands, theplurality of consecutive frequency bands being distributed along a Barkfrequency scale.
 12. The method of claim 1, wherein determining theenergy level in each of a plurality of frequency bands of the digitalaudio signal comprises determining the energy level in each of pluralityof bands, the plurality of frequency bands being distributed along aBark frequency scale.
 13. The method of claim 12, further comprisingdetermining the percentage of energy in a low frequency band of a totalenergy in a present frame, wherein the low frequency band includes atleast one Bark scale band, and wherein determining that the at least oneratio is not consistent with a voice signal further includes determiningthat the percentage of energy in the low frequency band is below athreshold.
 14. The method of claim 12, further comprising: determiningan average long term energy that is an average of total energy in eachof a plurality of successive frames; determining that a present frameenergy exceeds the average long term energy.
 15. The method of claim 12,further comprising: generating a filterbank index by indexing aplurality of the Bark scale bands by ranking them in order of energymagnitude; generating a histogram from the filterbank index based on theenergy in each of the indexed Bark scale bands.
 16. A two-way radiodevice, comprising: a receiver path, comprising: a signal source thatprovides a digital audio signal; a windowing component that formats thedigital audio signal into a sequence of frames; and an acoustic feedbacksuppressor that determines an energy level in each of a plurality offrequency bands of the digital audio signal, calculates at least oneratio of a signal energy level of a first frequency band of theplurality of frequency bands to a signal energy level of a secondfrequency band of the plurality of frequency bands, determines that theat least one ratio exceeds a threshold, determines, based at least inpart upon determining that the at least one ratio is not consistent witha voice signal, that feedback is occurring in one of the plurality offrequency bands caused by a transmitting two-way radio, and applies again reduction to the frequency band in which feedback is occurring tosuppress the feedback.
 17. The two-way radio of claim 16, wherein theacoustic feedback suppressor further dynamically sets the thresholdbased on a present pitch of the audio signal.
 18. The two-way radio ofclaim 16, wherein the acoustic feedback suppressor is in the receiverpath of the two-way radio.
 19. The two-way radio of claim 16, whereinthe acoustic feedback suppressor determines the energy level in each ofthe plurality of frequency bands of the digital audio signal as theenergy in each of a plurality of bands of a Bark scale.
 20. The two-wayradio of claim 16, wherein the acoustic feedback suppressor furtherdetermines the percentage of energy in a low frequency band of a totalenergy in a present frame, wherein the low frequency band includes atleast one Bark scale band, and that the percentage of energy in the lowfrequency band is below a threshold.
 21. The method of claim 1, whereinthe feedback is regenerative feedback caused by the receiving two-wayradio being sufficiently close to the transmitting two-way radio, and aspeaker output of the receiving two-way radio is received at amicrophone of the transmitting two-way radio.
 22. The two-way radio ofclaim 16, wherein the feedback is regenerative feedback caused by thereceiving two-way radio being sufficiently close to the transmittingtwo-way radio, and a speaker output of the receiving two-way radio isreceived at a microphone of the transmitting two-way radio.
 23. Themethod of claim 1, wherein the gain reduction is applied by an acousticfeedback suppressor operating in at least one of: the receiver path ofthe receiving two-way radio; and a transmit path of the transmittingtwo-way radio.
 24. The method of claim 1, wherein the feedback isacoustic feedback.
 25. The two-way radio of claim 16, wherein thefeedback is acoustic feedback.
 26. The method of claim 1, wherein thefirst frequency band and the second frequency band of the plurality offrequency bands are sub-bands of the plurality of bands.
 27. The two-wayradio of claim 16, wherein the first frequency band and the secondfrequency band of the plurality of frequency bands are sub-bands of theplurality of bands.